The present invention relates generally to hydrothermal apparatus and methods for growing single crystals, and particularly, to guiding flow and filtering solution within hydrothermal growth processes.
There exists a great demand for single crystals, such as .alpha.-quartz, of high purity and crystalline perfection for frequency control applications in the radio, television, telecommunications, and electronics industries. Hydrothermal techniques have been used to grow high-perfection single crystals for these and other applications.
To summarize the conventional process, a near-insoluble crystal nutrient starting material is immersed in an aqueous solvent within a closed-volume, steel autoclave. The contents are super-heated, thereby expanding the solvent to fill the entire autoclave, pressurizing the contents, and inducing dissolution of the crystal nutrient in a first zone of the autoclave. A temperature gradient is applied to encourage convective flow or the nutrient-laden solution from the first zone to a second zone having a different temperature. The solution reaches its saturation point and the crystal nutrient precipitates out in the second zone. Racks of seed crystals are usually provided in the second zone as nucleation points in order to minimize random self-nucleation of the nutrient. The reader is directed to the prior art on hydrothermal crystal apparatus and methods for greater details than disclosed in the following paragraphs. See, e.g., Sullivan, U.S. Pat. No. 2,994,593 (Aug. 1, 1961); Kolb, U.S. Pat. No. 3,271,114 (Jun. 15, 1964); V. A. Kuznetsov and A. N. Lobachev, "Hydrothermal Method for the Growth of Crystals", Soviet Physics --Crystallography, vol. 17, no. 4 (January-February, 1973); R. A. Laudise, "Hydrothermal Synthesis of Crystals", Special Report, C&EN, Sep. 28, 1987, pp. 30-42.
Several factors affect the quality of hydrothermally grown crystals and the efficiency of crystal production. They include: impurities present in the starting materials or introduced into the solution by corrosion of the autoclave vessel and baffle; the quality of the seed crystals used for nucleation; flow patterns of the dissolved nutrient within standard autoclave set-ups; baffle designs; and dissolved nutrient within standard autoclave set-ups; baffle designs; and temperature and pressure fluctuations affecting uniformity of growth rates. See, e.g., Balascio et al., "Factors Affecting the Quality and Perfection of Hydrothermally Grown Quartz", Proc. 34th Annual Symposium on Frequency Control, 65-71 (1980); Klipov et al., "Influence of Convective Flows on the Growth of Synthetic Quartz Crystals", Proc. 45th Ann. Symp. Freq. Control, 29-36 (1991); Johnson et al., "Experimental Determination of the Relationship among Baffle, Temperature Difference and Power for the Hydrothermal Growth of Quartz", 43rd Ann. Symp. Freq. Control, 447-458 (1989).
Various efforts have been directed at controlling these factors. For example, inert or noble metal linings have been proposed for reaction vessel walls, baffles, seed racks and seed clips. Such linings better withstand the solvent's corrosive effects and minimize formation of iron and other metal silicates, thereby reducing inclusions within the final crystals. Etched, dislocation-free seed may also be used on which to grow low-dislocation crystals for high-frequency applications. See, e.g., Barns et al., "Dislocation-free and low-dislocation quartz prepared by hydrothermal crystallization", J. Crystal Growth 43, 676-686 (1978); Croxall et al., "Growth and Characterization of High Purity Quartz", Proc. 36th Ann. Symp. Freq. Control, 62-65 (1982); Armington et al., "The Growth of High Purity, Low Dislocation Quartz", Proc. 38th Annual Symp. Freq. Control, 3-7 (1984). Greater amounts of aqueous solution may be used in low-pressure hydrothermal crystal growth processes to produce high-quality crystals of silicon-free materials. See, e.g., Caporaso et al., U.S. Pat. No. 4,579,622 (Apr. 1, 1986).
Persons skilled in the art of hydrothermal crystal growth have also recognized that dissolved nutrient and solvent flow back and forth randomly, even turbulently, between dissolving and growth zones of an autoclave. This problem is inherent with the use of conventional baffles-flat perforated disks-which tend to promote random mixing rather than convection. See, e.g., Annamalai et al., "Effect of Convective Baffle & Lithium Nitrite and Lithium Nitrite Dopants on Hydrothermal Growth Rate of Quartz Single Crystals", 21 Indian J. Tech. 425-430 (October 1983). Random flow promotes wasteful nucleation of dissolved nutrient on autoclave walls and components, and the solvent's corrosion of these components which are typically made of steel. As well, significant amounts of impurities --including iron or aluminum silicates and gas bubbles--accumulate to form inclusions in the crystals grown. Furthermore, crystal formation is non-uniform throughout the autoclave's growth zone, according to the influence of concentrative and convective flows. Klipov et al., Proc. 45th Ann. Symp. Freq. Control, 29-36 (1991).
Annamalai et al. discussed the desirability of a better mechanism of fluid flow within hydrothermal systems: i.e., convective fluid flow is preferred over random mixing of cooler and hotter fluids within a hydrothermal system. They discuss the merits of a "convection baffle" of their design in promoting convective flow, but disclose no specific details of their baffle design. They experimented with adding syphons and extending tubes to their baffle to minimize mixing at the fluid exchange boundary, but soon abandoned these fixtures as significantly impeding flow rate. Annamalai, 21 Indian J. Tech. at 426, col. 2.
In U.S. Invention Reg. No. H580 (Feb. 7, 1989), "Method and Apparatus for Growing High Perfection Quartz", Vig imposed "forced convection" on a hydrothermal system to reduce crystal imperfections and inclusions. A separate filter vessel and pump are attached to a crystal growth autoclave to circulate solution through the autoclave in one direction-from dissolving zone to growth zone to filter vessel back to dissolving zone-in a continuously recirculating pathway. However, use of a second autoclave as a filter vessel complicates and lengthens the hydrothermal process, and adds significantly to the cost of hydrothermal crystal growth. The filter vessel increases the volume of the system, thereby making it more difficult to maintain constant pressure and to control zone temperatures. Furthermore, forced convection tends to carry contaminant particles along with the crystal nutrient solution, increasing the risk of crystal inclusions. Vig's device provides a filter to remove contaminant particles from nutrient-depleted solution, but only after the dissolved nutrient and contaminants have already passed through the growth chamber.
Therefore, an unfulfilled need remains for a simple, cost-effective way to improve flow patterns in conventional hydrothermal crystal growth apparatus, thereby improving crystal quality and the efficiency of crystal production such that wasteful crystal deposition is minimized. There also remains a need for reducing the level of impurities present in the nutrient-laden solution during crystal growth, which reduces the inclusion density and increases the degree of crystal perfection.